1. Date Stamp. One of the quickest ways to ruin a good photo is to have the date stamped in big blocky red or orange letters in the corner. Turn it off. If your camera has the date set up correctly, the image will file will have an accurate creation date.
2. Sound Effects. One of the quickest ways to ruin a beautiful moment for those around you is to force everyone in ear shot to listen to the beep beep beep and simulated click of your camera as it struggles to take the photo that probably won't turn out anyway. Turn them off. There are all sorts of little lights and information on the displays that already tell you that you've focused, turned on your camera, etc.